Preheat oven to 350°F.
Line two baking sheets with parchment paper.
Beat butter, granulated sugar, and brown sugar together until smooth and creamy.
Add egg yolks and vanilla, mixing again until fully incorporated.
Add flour, cornstarch, and salt.
Mix until the dough comes together.
Refrigerate dough for 30 minutes.
Roll dough into 1-inch balls and place on prepared baking sheets.
Use your thumb or the back of a 1/2 teaspoon to gently press a well into each cookie.
Spoon about 1/2 teaspoon jam into each well.
Bake for 12–15 minutes, just until the bottoms look lightly golden.
Cool on the baking sheet for a few minutes, then transfer to a wire rack to finish cooling.
